A Metropolitan Area Transport device, developed on the basis of devices of the traditional Synchronous Digital Hierarchy (SDH), has now incorporated such data access techniques as Ethernet and Asynchronous Transfer Mode (ATM), which is typically represented by the device of multi-service transport platform (MSTP). In a Metropolitan Area Transport device, the protection methods, such as Multiplex Section Protection (MSP), Sub-Network Connection Protection (SNCP), Path Protection (PP), and etc, may be adopted by an SDH service to implement the protection of network level. However, there is no protection method special for the Ethernet data services transported by the Metropolitan Area Transport Device, and the protection thereof has to rely on the methods adopted by the SDH service.
Along with the increasing significance of applications of data services in a Metropolitan Area Transport Network, the demand for higher security and reliability of some important data services has to be satisfied. The traditional SDH protection methods, however, can not serve this purpose, for they provide protection only when there is a failure in an intermediate link or at an intermediate node via which the service is transported rather than when there is a failure at the node of termination of a link or in the links connected with the data devices. FIG. 1 shows the method of traditional SDH protection:
Suppose that a service is uploaded to the Metropolitan Area Transport Network at Network Element A, then transported through Network Element E, and downloaded at Network Element D. In such a case, a traditional SDH protection method only works in the following three situations: there is a failure in the link between Network Element A and Network Element E, a failure in the link between Network Element E and Network Element D, or a failure in Network Element E. In FIG. 1, such a protection link is marked with a dotted arrowhead, which can not work if Network Element D fails or there is a failure in the data interface of the services in the downlink of Network Element D.
In the Ethernet, the protection function in case of a failure in a Network Element or in the link between a Network Element and a data device connected with the Network Element could be implemented via the existing Ethernet Layer Two (L2) switching service, of which the scheme is shown in FIG. 2: data units in Network Element A, Network Element C and Network Element D are all configured with the Ethernet L2 switching service while Network Element C and Network Element D are connected to Network Element A, respectively, via different routes; in normal conditions, the interface of Network Element D serves as a work interface (a normal service interface, or the interface being protected), and the interface of Network Element C serves as a protection interface (a protection service interface, in charge of protecting the work interface), the work interface and the protection interface are connected to the data network, respectively, via identical devices or different devices. In normal conditions, to ensure the correctness of the service, it is required that the data devices connected with the protection interface shall not transport any message to the protection interface while messages have to be transported to the work interface. If there is a failure in Network Element D, in the link of the work interface or in the data device connected with the work interface, the data device connected with the protection interface will detect the failure; thereby switch the service to the protection interface to perform the protection function.